Doc: Allow to easily link to qt_target_qml_from_lottie()

Add qt_target_qml_from_lottie() as an alternative \keyword to the
CMake command page, replacing \keyword qt6_target_qml_from_lottie,
and update the example that references it to use the new keyword
directly.

Pick-to: 6.12
Change-Id: I4ff80728ff045b9641722c90baf9ebe322def3ec
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>